A Dictionary of Music and Musicians/Leading Note
LEADING NOTE (Fr. Note sensible; Germ. Leitton). In modern music it is absolutely indispensable for all harmonic progressions to have an appreciable connection with a tonic or keynote, and various lines converge to indicate that note with clearness; among these an important place is occupied by the Leading Note, which is the note immediately below the keynote, and separated from it by the smallest interval in the system, namely a semitone. Helmholtz has pointed out that in actual relationship to the tonic it is the most remote of all the notes in the scale, since the supertonic, which also appears to be very remote, at least comes nearer in being the fifth to the dominant, while the leading note is only the third. For this reason, and also from its not being capable of standing as a root note to any essential diatonic chord in the key, it seems to have no status of its own, but to exist mainly as preparatory to the tonic note, for which, by reason of its close proximity, it seems to prepare the mind when it is heard; and the melodic tendency to lead up to the most important note in the scale is the origin of its name.
In many scales, both of civilised and barbarous peoples, it has found no place. In most of the mediæval ecclesiastical scales, as in the Greek scales from which they were derived, the note immediately below the tonic was separated from it by the interval of a whole tone, and therefore had none of the character of a leading note; but as the feeling for tonality gained ground in the middle ages hand in hand with the appreciation of harmonic combinations, the use of the leading note, which is so vital to its comprehension, became more common. Ecclesiastics looked upon this tampering with the august scales of antiquity with disfavour, and Pope John XXII passed an edict against it in 1322; consequently the accidental which indicated it was omitted in the written music: but the feeling of musicians was in many cases too strong to be suppressed, and it seems that the performers habitually sang it wherever the sense of the context demanded it, nor do we learn that the ecclesiastics interfered with the practice as long as the musicians did not let the world see as well as hear what they were doing. Notwithstanding this common practice of performers, the scales maintained their integrity in many respects, and there resulted a curious ambiguity, which is very characteristic of mediæval music, in the frequent interchange of the notes a tone and a semitone below the tonic. Musicians were long beguiled by the feeling that the true scales should have the note below the tonic removed from it by the interval of a tone, and that it was taking a liberty and pandering to human weakness to sharpen it; and the clear realisation of those principles of tonality upon which modern music is based was considerably retarded thereby, so that works both vocal and instrumental are characterised by a vagueness of key-relationship, which the use of the leading note alone can remove, till far on into the seventeenth century; by the time of Bach and Handel however the ancient scales had been fused into the major and minor modes of the modern system, and the leading note assumed the office it has ever since occupied. The gradual realisation of the importance of the leading note and the influence it had upon the development of modern music is traced in the article Harmony, and reference may also be made to chap, xiv of the Third Part of Helmholtz's great work on 'The Sensations of Tone,' etc.